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The tropical oceans are bursting with glare and lightning from colorful reef fish and contain many more species than the cold waters of the ocean found at high latitudes. This well-known "latitudinal diversity gradient" is one of the most famous models in biology, and scientists have stunned its causes for over 200 years.
A common explanation for the gradient is that warm reef environments serve as spots for species formation. But a new study analyzing the evolutionary relationships among more than 30,000 fish species concludes that the fastest rates of species formation occurred at the highest latitudes and in the coldest ocean waters.
According to the new study, published in the journal Nature the fish of the polar ocean have formed twice as fast a new species as the average species of tropical fish.
Lauren Sallan of the University of Pennsylvania collaborated with a team from the University of Michigan who led the work, compiling one of the largest sets of data on fossil fish at this time. day to calibrate the evolutionary tree of fish
"By using fossils of the early ancestors of modern fish, we can have the date of their appearance on the face of the earth," says Sallan, assistant professor Martin Meyerson in interdisciplinary studies at Penn's Land Department and Science of Science. 39, environment at the School of Arts and Sciences. "Fossils help us determine the rate of evolution of different groups of fish."
"These findings are both surprising and paradoxical," says Daniel Rabosky, evolutionary biologist at the University of Michigan -Ann Arbor, lead author of the study. Professor in the Department of Ecology and Evolutionary Biology of UM and Associate Curator at the Museum of Zoology of UM
"A number of assumptions explain extreme tropical diversity as the result in faster rates of species formation. ", says Rabosky. "Our results are counterintuitive and unexpected because we find that speciation is the fastest in geographic areas with the lowest species richness."
The authors admit that they can not fully explain their inconsistent findings with the idea that the tropics have an evolutionary cradle for marine fish diversity. The results also raise questions as to whether the rapid speciation of the ocean by the cold testifies to a recent and continuing expansion of marine diversity.
Common sense suggests that a high rate of new species formation will result in impressive levels of biodiversity. But it depends on the number of newly formed species that survive and how much they extinguish. And extinction rates could not be corrected by the methods used in this study.
"The number of species found in a region," says Rabosky, "is largely a balance between the rate of formation and the rate at which extinction eliminates them. The rapid speciation of fish in the cold oceans of high latitudes we have documented will only increase diversity if it is usually higher than extinction.
"Extinction is the missing piece of this puzzle, but it's the hardest thing to understand. , "he says." We now use both fossils and new statistical tools to try to understand what extinction could have done in the polar regions and the tropics. "
In the study, Michigan researchers worked with Sallan and colleagues from seven other institutions to test the widespread assumption that species formation rates are faster in the tropics by examining the relationship between latitude, species richness and the rate of new species. training among marine fishes.
They assembled a calibrated scalable tree of 31,526 species of ray-finned fish and then focused their analysis on marine species around the world.
Genetic data was available for more than one-third of the fish species analyzed in the study. . The evolutionary tree was calibrated using a database of 139 fossil taxa, which were painstakingly collected by Sallan as well as by undergraduates and students. graduate studies in his laboratory. What we see in the fossil record, "says Sallan.
An evolutionary tree, also known as a phylogenetic tree, is a branched diagram showing inferred evolutionary relationships between various species. The tree assembled for this project is one of the largest time-phased phylogenetic trees ever created for a group of animals.
The researchers estimated the geographical areas of most marine species, including all genetic species The data Then they used complex mathematical and statistical models to estimate the rates at which different groups of fish divide into new species.
"Computer challenges to analyze these types of data are quite extreme, "says Michael Alfaro, co-author of the evolutionary biologist's study at the University of California at Los Angeles. s of the study required the equivalent of thousands of desktops running continuously for several months,
Some of the highest rates of formation of new species occurred in the fish of Antarctic ice and their parents. Other temperate and polar groups with exceptionally high speciation rates include snailheads, killer whales and rockfishes.
Three of the largest groups of coral-reef-associated fish – wrasses, damselflies and gobies – had low to moderate species formation rates. "The fact that coral reefs support many more fish species than the polar regions despite these lower rates may have a lot to do with their long history of connectivity and their ability to serve as refuges," says the co-author Peter Cowman. Center of Excellence Council for Coral Reef Studies, and previously from Yale University. "Our research certainly describes the diversity of coral reefs in a new light."
"I think it's a really exciting result," says Sallan. "The signal is strong and appears in all the analyzes done.It is so unexpected that you really have to revise your assumptions about how biodiversity works."
The authors of the article Nature in addition to Sallan, Rabosky, Alfaro and Cowman, are Jonathan UM Chang, O Pascal Title, and Matt Friedman; Kristin Kaschner from the University of Freiburg; Cristina Garilao from the GEOMAR Helmholtz Center for Ocean Research; Thomas J. Near of Yale University; and Marta Coll of the Institute of Ocean Sciences in Barcelona
The work was supported in part by a DEB-1256330 and a DDIG grant from the National Science Foundation and by a grant from the David and Lucile Foundation Packard.
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